1. Field of the Invention
The present invention relates to an improvement of a lens barrel provided in an optical instrument such as a camera and having conductivity and antistatic properties for preventing electronic circuit failure inside the instrument and instrument malfunction caused by charging.
2. Related Background Art
A lens barrel is generally formed by molding a plastic resin from a viewpoint of miniaturization, weight saving, and thickness reduction. Further, a lens barrel main body needs to have conductivity for preventing instrument failure caused by external charging. That is, a solution is adopted in which a carbon fiber as a conductive material is mixed with a resin material used for the lens barrel, to thereby impart the conductivity to the lens barrel main body and to release charge to the lens barrel. Examples of the instrument failure include malfunction (such as image distortion) caused by falling of external stored charges to a terminal connected to an electrical circuit inside a digital camera. However, the mixing of the carbon fiber causes problems such as significant reduction in moldability (fluidity) of the resin material, increase in abrasion of a metal mold involved in deterioration of the fluidity, and huge cost for mold correction. The mixing of the carbon fiber also leads to weight increase of the lens barrel, thereby basically inhibiting miniaturization, weight saving, and thickness reduction.
Further, a resin composition having higher conductivity has been proposed recently, which is prepared by mixing a conductive material represented by the carbon fiber or the like with a resin. Examples of mixing methods include: a method of increasing a mixing amount of the conductive material such as a carbon fiber; and a method of combining a plurality of conductive materials. Increasing the mixing amount of the conductive material provides high conductivity, but also causes problems in deterioration of moldability (fluidity) of the material and reduction of mechanical properties such as impact strength. On the other hand, examples of the method of increasing conductivity by combining a plurality of conductive materials include a technique of combining a carbon fiber with carbon black. The combining improves the conductivity, but does not provide sufficient moldability (fluidity) or mechanical properties.
As described above, a resin material having excellent moldability or the like is hardly obtained unless a carbon fiber content is reduced or the carbon fiber is eliminated. In other words, combination of the conductivity and the moldability is difficult.
The mixing of the carbon fiber with the resin material to impart conductivity described above, as a solution to instrument failure, causes problems such as significant reduction in moldability (fluidity or the like during molding) of the resin material, increase in abrasion of the metal mold involved in deterioration of the fluidity, and huge cost for mold correction. The mixing also leads to weight increase of the lens barrel.
Thus, the mixing of the carbon fiber is far from the best method for miniaturization, weight saving, thickness reduction, and cost reduction. That is, it has been desired to impart conductivity by eliminating or reducing use of the carbon fiber conventionally used, without losing the moldability, and using a replacement of the carbon fiber causing small mold abrasion.
Further, a resin containing a glass fiber or a carbon fiber has poor fluidity, and thus, has problems such as difficulties in reducing the thickness of lens barrel components and in providing given rigidity when the content of the glass fiber or the carbon fiber is reduced to enhance the fluidity.
Further, the glass fiber or the carbon fiber orients owing to a resin flow, causing anisotropy in shrink ratio of the resin. Thus, high dimension accuracy was also hardly obtained in the lens barrel components.
Further, the glass fiber or the carbon fiber abrades a metal mold during molding and shortens the life of the metal mold. For example, the lens barrel components constituting a zoom lens barrel unit are provided with a fitting portion requiring high precision. Thus, the metal molds of all components must be produced again when any of the metal molds is abraded in the various lens barrel components constituting the lens barrel unit. Thus, the cost of the metal molds also becomes a concern.